The method of detecting an insulation defect at a winding is conventionally known.
For example, Japanese Patent Laying-Open No. 64-9360 discloses the method of detecting the presence and location of a pinhole in a winding by reducing the pressure in a vessel while constant voltage is applied to the winding of the rotating electric machine to cause partial discharge.
Further, Japanese Patent Laying-Open No. 2003-248028 discloses the method of identifying the location of a coil defect by means of a liquid level sensor, including the steps of gradually supplying noncorrosive and conductive inspection liquid into an inspection tank, applying voltage across an electrode and coil to identify the location of the coil defect on the basis of the height of the inspection liquid surface measured by the liquid level sensor at the point of time when electrical conductivity is detected across the electrode and coil.
In addition, Japanese Patent Laying-Open No. 9-43301 discloses an insulation testing method including the steps of applying surge voltage to the coil, and detecting whether glow discharge is generated from the coil or not.
In detecting an insulation defect, identification of the location of the defect, in addition to the presence of such a defect, is important. Although the three documents set forth above disclose identification of a defective region based on a concept different from that of the present invention, none of the approaches are necessarily sufficient from the standpoint of identifying the accurate position.